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Fuel Pressure Problems

  • eswallie
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5 months 2 days ago #19192 by eswallie
Fuel Pressure Problems was created by eswallie
I am replacing an old 912 ULS with a brand new one. On the old engine, low fuel pressure was a continual problem with several electric and mechanical fuel pumps changed. Never was completely solved.
Installed the new engine with everything new except the electric fuel pump. For the first 1/2 hour the fuel pressure was 6#, and then it gradueally declined (mechanical pump). Now the electric pump has declined until I get about 2.5# out of both. The check valve is currently in line with the electric pump, but I noticed in the fuel schematic that Rotax shows the check valve in parallel. Which is correct? Could this be the problem with the low fuel pressure?
Ernie

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5 months 1 day ago #19215 by PeterKl
Replied by PeterKl on topic Fuel Pressure Problems
Make it a parallel setup. The check valve must be installed so it allows the fuel to bypass the electric pump in case of a failure. Orientation of that valve is key! Assume that the mechanical pumps pulls the fuel from the tank while the electric pump is all chokablock. In that case the fuel flow must go through the check valve which is in parallel to the electric pump.
If the electric pump works all fine the check valve remains closed and hinders the fuel to go crazy in a circle electric pump, valve, electric pump,....
I cannot see any benefit of a serial setup.

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5 months 1 day ago #19217 by Kevin Stewart
Replied by Kevin Stewart on topic Fuel Pressure Problems
If you have a vapour return line to the tank then you may not have the correct size restrictor hence the low pressure.

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5 months 1 day ago #19223 by eswallie
Replied by eswallie on topic Fuel Pressure Problems
This is the current setup. www.screencast.com/t/mweCPRAR2bm
Ernie

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5 months 22 hours ago - 5 months 21 hours ago #19226 by Roger Lee
Replied by Roger Lee on topic Fuel Pressure Problems
Check valves, fittings and the hose diameter will affect the flow and pressure. You will have friction loss because of the hose and fittings which is a reduced flow at the end.You may also need to increase the electric fuel pump size. I just fixed a Sting with low fuel pressure by going to the next size Facet pump. It had a 3 - 4.5 psi 30 gph and I went to a 4 - 7 psi at 32 gph. His pump setup was in series and max pressure went from 1.5 psi on a hot day to 5.7 psi with both pumps on. Just food for thought.
Take a look at the Installation Manual 74-00-00 page 4 on the diagram.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell
Last edit: 5 months 21 hours ago by Roger Lee.

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